This study was designed to show the effect of different primary standards and antisera on the variability of concentrations of apolipoprotein A-I (ApoA-I) in plasma and to document the usefulness of reference sera as a secondary standard in the electroimmunoassay of this apolipoprotein. We compared the reactivities of two separate ApoA-I samples and a preparation of high-density lipoprotein (HDL3) with seven different antisera to ApoA-I. The antisera reacted differently with each of the three standards, as shown by the different slopes of their standard curves. The resulting inconsistencies in the ApoA-I values for unknown plasma samples were shown to be corrected by the use of a reference serum. When ApoA-I analyses were carried out with the reference serum as a standard, all seven antisera gave statistically indistinguishable results for concentrations of plasma ApoA-I. The use of reference sera thus represents the simplest and most convenient means of standardizing the electroimmunoassay for ApoA-I.